Protective device comprising a plurality of serially connected spark gaps

ABSTRACT

1. In a protective device for protecting a facility from damage due to an unexpected increase in the normal voltage on said facility, said protective device comprising a plurality of spark gaps, said gaps being serially connected, in a chain, across said facility; a corresponding plurality of resistors, one of said resistors being connected in parallel across each of said gaps, and in series with the rest of said resistors across said facility, to control the potential developed across each gap; a plurality of capacitors directly connecting alternate electrodes of each gap to opposite ends of said device whereby any unexpected voltage increase which occurs is immediately and directly coupled by said capacitors to each of said gaps.

United States Patent 11 1 3,683,234 Rodewald Aug. 8, 1972 [54] PROTECTIVE DEVICE COMPRISING A 2,61 1,108 9/1962 Rydbeck ..3l5/36 PLURALITY ()F SERIALLY 3,510,726 5/1970 Harder ..3l5/36 CONNECTED SPARK GAPS Primary Examiner-Nathan Kaufman [72] Inventor: Arnold Rodewald, Sterngrubenweg mmmey |(urt Kelman l 16, Riehen near Basel, Switzerland 22 Filed: May 13,1969 EXEMPLARY CLAIM [2]] App]. No; 824,125 1. ln a protective device for protecting a facility from damage due to an unexpected 1ncrease 1n the normal voltage on said facility, said protective device com- Fm'elgn Apphcanon Pnonty Data prising a plurality of spark gaps, said gaps being seri- May 17,1968 Switzerland ..7368/68 ally a said m respondmg plurality of res1stors, one of sa1d resistors 52 us. c1. ..315/238, 315/245, 315/324, f f Parallel 9 /189 and 1n serles w1th the rest of sa1d reslstors across sa1d 51 I cl 05b 37 02 facility, to control the potential developed across each nt. gap; a plurality of capacitors directly connecting [58] new of Search 315/36 ternate electrodes of each gap to opposite ends of said 315/187, 189, 228, 229, 231, 238, 324 device whereby any unexpected voltage increase which occurs is immediately and directly coupled by References Cited said capacitors to each of said gaps.

UNITED STATES PATENTS 1 Clai 1 Drawing Figure 2,611,107 9/1962 Rydbeck ..3l5/36 PATENTEDAus a m: 3,5 3; 234

1 NVENTOR. ARNOLD RODEWALD AGENT PROTECTIVE DEVICE COMPRISING A PLURALITY OF SERIALLY CONNECTED SPARK GAPS For certain purposes spark gaps that have an impulse ratio smaller than 1 are required. Impulse ratio is understood to be the ratio of the flashover voltage under an impulse voltage load to the flashover voltage under a steady d.c. or an ac. voltage load. Such spark gaps are needed for instance in overvoltage diverters or as components of a multistage Marx impulse generator circuit. Generally speaking such spark gaps can be used to suppress overvoltages that are superimposed upon a working voltage, a working voltage in this context being understood to be a d.c. voltage or an industrial frequency ac. voltage.

For an assessment of the serviceability of spark gaps the following criteria are relevant:

a. Flashover must definitely not occur when the working voltage is the only load,

b. The ratio of the flashover voltages under an impulse voltage load and the working voltage should be as low as possible. This criterion is of particular importance because the dimensioning of the insulation is determined by the magnitude of the overvoltages. In other words, if the overvoltages are limited, for instance by a protective device, the required dimensioning of the insulation may possibly turn out to be more economical than would be the case if the full magnitude of the overvoltage had to be taken into account.

. The flashover delay of the spark gap, i.e., the time which elapses between the appearance of an overvoltage and potential breakdown between the electrodes should be a minimum. A long flashover delay would still expose the insulation of the apparatus which is to be protected to a high overvoltage before the protective action of the spark gap became effective.

The following spark gap arrangements are known in the art to provide an impulse ratio smaller than I.

1. Uniform field spark gaps, such as sphere gaps or plate gaps which have a pin insulatedly embedded in one of the electrodes. The pin is connected to the main electrode i.e., the sphere or plate exclusively through a high ohmic resistance. When stressed by the working voltage no substantial potential differences can develop between the pin and the main electrode and the entire arrangement works as a unform field gap. However, if an impulse voltage is applied to the two electrodes, then the delay in transmitting the charge to the pin may result in the development of considerable potential difference between the pin and the main electrode. In the case of an impulse voltage the arrangement therefore functions like a point-plate gap and flashover develops after the delay characteristic of uniform field spark gaps following the appearance of the impulse voltage. An impulse factor of about 0.8 can be achieved with an electrode arrangement of this kind.

2. Multiple spark gaps with a linear voltage distribution of the working voltage across the several gaps, said linear distribution being achieved by resistance control.

When the multiple gap is stressed by an impulse voltage the resistance control becomes ineffective. In such a case the voltage distribution in the multiple spark gap is determined exclusively by the capacitances between the several electrodes and the stray capacitances of these electrodes to the two voltage-carrying conductors, e.g., high voltage electrodes and earth electrodes. This voltage distribution is exponential across the gap. Owing to this non-linear voltage distribution when an impulse voltage appears, some of the constituent gaps are over-stressed. Breakdown occurs and the other constituent gaps are then over-stressed so that in this way flashover develops across the entire multiple gap. 3. Apart from these passive electrode arrangements which have an impulse factor below 1 electrode arrangements have been proposed in which flashover can be induced below the working voltage level by a trigger device. The above-mentioned overvoltage protection then functions to provide part of the overvoltage as a control signal for the trigger device which usually initiates breakdown in the gaps by the generation of one or more auxiliary sparks. The major difference from the previously mentioned arrangement resides in that flashover in such a controlled spark gap depends upon the proper functioning of the trigger device. If any one component of the trigger device should fail, then malfunctioning of the entire arrangement is likely.

The present invention relates to a multiple spark gap comprising a chain of serially connected constituent gaps, in which a resistor is connected across each constituent gap for controlling the potential of the constituent electrodes, and these resistors are themselves connected in series. According to the invention, the constituent electrodes in such a multiple gap are alternately capacitively coupled to one and the other end of the chain. The required breakdown resistance to the working voltage can be assured in conventional manner by resistor control.

Details of the invention will be understood from the following description of an embodiment shown in the drawing.

The drawing represents a multiple spark gap comprising five constituent gaps, 1 and 8 being the spark gap terminals, 2 to 7 electrodes and 9 resistors, whereas 14 symbolizes the stray capacitances between the electrodes23,3-4,4-5,56and67. The constituent gaps are fonned between the electrodes 2 and 3, 3 and 4, and v5, 5 and 6, and 6 and 7. In the illustrated embodiment all these electrode gaps are of the same length. So long as only the working voltage appears between the terminals 1 and 8 the potential distribution along the electrode chain is controlled by the resistors. According to their number and assuming that the resistors 9 are all equal, the potential across each gap will be one-fifth of the working voltage. However, if an impulse voltage appears between the terminals 1 and 8, then the potential distribution ceases to be determined by the resistors and the effective capacitances in the arrangement take over control. In the illustrated embodiment, and as proposed by the invention, the electrodes 4 and 6 are coupled to the terminal 1 by a conductor 10 and capacitors 11, whereas the electrodes 3 and 5 are coupled to the other terminal 8 by a conductor 13 including capacitors 12. If the capacitance of the capacitors 11 and 12 substantially exceeds the stray capacitances 14 indicated by chain lines between the constituent electrodes of the multiple spark gap, then the potential distribution via the capacitors 12, 14, 11 will be such that substantially the full impulse voltage across the terminals 1 and 8 will simultaneously also appear across the electrode pairs 2 and 3, 3 and 4, 5 and 6 as well as 6 and 7. The breakdown resistance to an impulse voltage therefore substantially depends upon the resistance to the impulse voltage of the gap between two constituent electrodes. In the embodiment shown in the drawing comprising five constituent gaps the resistance to an impulse voltage will therefore be roughly one-fifth of the resistance of the electrode system to a working voltage. The described principle of construction permits the impulse factor to be reduced as desired by a suitable fine subdivision of the multiple spark gap.

If the described multiple spark gap is now assessed by reference to the above mentioned criteria, the following picture arises:

point a. The described arrangement has the advantage that it permits the flashover voltage under impulse stress to be adjusted independently of the flashover voltage under d.c. or ac. voltage stress, simply by providing a suitable number of constituent gaps, as has been described. Moreover, if the number of constituent gaps is fixed, it is still possible by variation of the capacitances of the control capacitors 11 and 12 to vary the impulse potential load will definitely not cause a flashover.

point b. The impulse ratio can be reduced as much as desired by sufficiently fine subdivision. Impulse factors of 0.1 and even less are readily obtainable.

point c. Since the impulse voltage appears across each constituent gap at the same time the delay in the development of flashover in the desired device is extremely short. This is a major advantage over multiple spark gaps hitherto known in which flashover usually develops consecutively from gap to gap, a process involving a corresponding overall flashover delay.

I claim:

1. In a protective device for protecting a facility from damage due to an unexpected increase in the normal voltage on said facility, said protective device comprising a plurality of spark gaps, said gaps being serially connected, in a chain, across said facility; a corresponding plurality of resistors, one of said resistors being connected in parallel across each of said gaps, and in series with the rest of said resistors across said facility, to control the potential developed across each gap: a plurality of capacitors directly connecting alternate electrodes of each gap to opposite ends of said device whereby any unexpected voltage increase which occurs is immediately and directly coupled by said capacitors to each of said gaps. 

1. In a protective device for protecting a facility from damage due to an unexpected increase in the normal voltage on said facility, said protective device comprising a plurality of spark gaps, said gaps being serially connected, in a chain, across said facility; a corresponding plurality of resistors, one of said resistors being connected in parallel across each of said gaps, and in series with the rest of said resistors across said facility, to control the potential developed across each gap: a plurality of capacitors directly connecting alternate electrodes of each gap to opposite ends of said device whereby any unexpected voltage increase which occurs is immediately and directly coupled by said capacitors to each of said gaps. 